Total field imaging probe

ABSTRACT

The invention relates to the principle of a probe to improve the quality of imaging logs in excavations, providing especially a good coverage of the wall 2 of the excavation by the beam of sensors 4 or electrodes. The basic component is constituted by a casing 6 moulded from a material of the elastomer type and made to allow it to be unfolded in the working position for the helically implanted zones of measurements 7, and applied against the wall 2. 
     The material of the zones 7 of measurements is selected as a function of the parameter to be measured, the immediate but non-limiting applications concerning microresistive and acoustic imaging. 
     The helical arrangement of the sensors 4 improves the coverage of the measurements with respect to conventional probes. Moreover, by eliminating the drilling mud 3 between the sensors 4 and the formation, this probe improves, in certain cases such as acoustic imaging for example, the response thereof and the quality of the measurements.

BACKGROUND OF THE INVENTION

The present invention has for its object a total field imaging probeenabling logs to be made over the whole periphery of the wall of a holeexcavated in the sub-soil.

The technical sector of the invention is the manufacture andexploitation in situ of tools or probes for making measurements of thecharacteristics of the soil in a bore-hole traversing geologicalformations and for remotely reading and analyzing these measurements.

One of the principal applications of the invention is the use thereoffor effecting imaging logs of wells making it possible to obtain a goodcoverage of the whole of the excavation, using different conventionalindividual measuring means, such as acoustic or microresistivemeasurements.

In fact, different systems and processes are known, of which certainhave formed the subject matter of Patent Applications and which relateto the same object and/or the same application as the present invention,but of which the technical solutions retained do not allow goodperformances and/or are difficult to implement.

In the following description and by way of definition, the generic term"electrical logging" or "log" designates the continuous recording ofphysical parameters of the formations encountered during excavation, asa function of the depth.

It will firstly be recalled that imaging logging is a novel conceptmaking it possible, from conventional individual measurements, topresent along an evolute of the wall of the borehole as a function ofthe depth, the response of this wall to a physical measurement, suchmeasurement having a vertical and horizontal definition of the order ofseveral tens of centimeters for usual logs.

This representation, on any displayable visual support and, furthermore,oriented in space, therefore constitutes an artificial image of theformations encountered.

Implementation thereof essentially necessitates:

an investigation with the aid of the parameter measured, allowing themaximum coverage of the wall of the well;

means for locating in space;

a processing of each measurement representing the amplitude of thisparameter in the form of a colour or a gradual range of greys; forexample, the high resistivities in white and the very low in black. Theamplitude of a sound wave, emitted by a sensor, after reflection by thewall of the well, may be processed in the same manner.

DESCRIPTION OF RELATED ART

For example, Patent FR 2 532 059 filed on Aug. 19, 1982 by the firmSCHLUMBERGER PROSPECTION ELECTRIQUE, describes a device for the visualpresentation of results of measurement, applicable to this type ofprocessing.

It is obvious that, in order to be representative of the formationsencountered, these measurements must investigate a maximum percentage ofthe wall of the well.

To that end, one possibility is to effect an acoustic imaging from arotating sensor serving as emitter and receiver, and mechanically posingno problems. The density of the data received depends only oncontrollable parameters:

speed of rotation;

ascensional speed;

number of scans per revolution.

This applies to any other measurement made from a rotating sensor, suchas for example the one described in Patent Application FR 2 448 621filed on Sep. 2, 1979 by the INSTITUT FRANCAIS DU PETROLE and entitled:"Probe with rotating shoe for effecting measurements in a borehole".

On the other hand, any measurement necessitating the contacting ofsensors or of shoes on the wall of a well poses mechanical problemsrecalled hereinafter.

In fact, in order to produce this contact, the probes usually use rigidor semi-rigid shoes constituting one side of a parallelogram, theopposite side being defined at its ends by one or two articulationslocated on the body of the probe, as described for an acoustic imagingin U.S. patent application Ser. No. 948 206 filed on Dec. 31, 1986 bythe firm SHELL INTERNATIONAL RESEARCH.

The coverage of the measurement depends on the size of the shoes, theirnumber and the diameter of the excavation. Furthermore, the descent oftool in the well, before the measurement, imposes that, in restposition, with shoes folded, the latter are contained within thediameter of the probe body and, in the acoustic use hereinabove, thefriction of the shoes on the wall generates many parasitic noises.

Within the framework of a current application, the coverage of a probewith an outer diameter of 41/2", for example in a hole of 81/2"(petroleum standards respectively equivalent to 114.3 mm and 216 mm),does not exceed 40%; it is no more than 28% in a well of 121/4" (311mm).

This servitude obliges the operators to make several recordings in orderthat the tool, having if possible rotated between two measurements,obtains a sufficient coverage of the wall.

In the domain of imaging for probes with electrodes of microresistivity,U.S. patent application Ser. No. 2 611 919 filed on Mar. 5, 1987 by thefirm PROSPECTION ELECTRIQUE SCHLUMBERGER is noted, concerning a loggingprobe with wide angular field: this probe makes it possible to doublethe coverage of the usual mechanical probe by allowing the implantation,at the end of each articulated arm, of two lateral flaps for measurementdisposed on either side of an axis parallel to the axis of the probe andoffset in height, in order to be able to position them on the probebody.

Such implantation theoretically makes it possible to obtain 100% ofcoverage in a well of 7" (non standard in open hole) and only 82% in81/2" with a probe whose outer diameter is 5". This, furthermore, leadsto complex, and consequently expensive, mechanics.

Another possibility for effecting a total field imaging, limiting themechanical problems, is to effect said imaging from a probe not touchingthe wall of the hole to be explored: however, the measurements collectedare automatically disturbed by the fluid, which is often mud, fillingthis hole and introduced between the sensors and the wall: the resultsare then difficult to interpret as, whether in the electrical oracoustic domain, the fluids used may constitute a parasitic element: inacoustics, the solid microparticles present in the mud diffract thesound wave and considerably reduce the amplitude of the wave received bythe sensor; in electrics, the low resistivity of the mud promotes apartial short-circuit of the lines of current at the output of theelectrodes.

Various studies have, of course, been noted, which have formed thesubject matter of Patent filings in order to limit these problemsassociated with the influence of the fluid, both in electrics and inacoustics, such as:

FR Patent Application 2 611 920 filed on Feb. 25, 1987 by the CNRS,which employs electric electrodes for detecting fractures located in aring at the center of the probe and associated with focussing andcollecting electrodes at the ends with correction means adapted to acton the potential of the emission electrodes in order to compensate thediffusion of the current in the fluid medium; in a variant, theprotection and emission electrodes have a movement of rotation about avertical axis of the probe.

U.S. patent application Ser. No. 935 422 filed on Nov. 26, 1986 by thefirm SHELL INTERNATIONAL RESEARCH, which describes an apparatus forproducing images of probe holes, with acoustic transducer, andcomprising adjustable variable gain amplification means in order toreduce the effects of false signals generated by the mud located in aring around the probe.

Apart from the problems, which are therefore of parasitical influence,of the introduced fluid which, despite the above projects, are notsolved in order to obtain a good interpretation of the measurements, itis necessary also to emphasize the difficulty of positioning the probesand of controlling their orientation since there is no bearing againstthe walls; this renders still more difficult implementation thereof inorder to have a reading of the wall which is as complete as possible.

A last possibility, on the one hand, for reducing the influence of thefluid of the above systems and, on the other hand, for simplifying themechanical means of the preceding ones, is to use probes with supplewall, which may then be applied against the wall; this possibility hasbeen developed in various applications, all with electrodes, of whichthe following two examples which have formed the subject matter ofPatent filings may be mentioned: U.S. Pat. No. 2 930 969 filed on May16, 1956 by Mr. BAKER, describes a supple casing open upwardly anddownwardly, like a floating anchor, and bearing electrodes on its outersurface, so that, hooked to a handling rod, it closes when it isdescended in the well and it opens by the speed of the fluid,penetrating in its upper part when it is raised.

Another U.S. Pat. No. 4 236 113 filed on Apr. 13, 1978 by Mr. WILEY forPHILIPS PETROLEUM, describes a probe comprising an inflatable body, onwhich are disposed electrodes in a horizontal plane and which comes intocontact with the wall when the probe is positioned in the desired place.

These systems effectively make it possible to reduce the mechanicalcomplexity of the articulated shoes, but do not solve the lack of totalcoverage of the wall of the well; moreover, the first system requires ahigh speed of rise of the probe and the second, on the contrary, thetaking of a measurement when stopped, as it then totally obturates saidwell.

Thus, none of these processes or devices makes it possible to solve theproblem to which the present invention brings, however, solutions: infact, the problem raised is that of being able to produce a probeallowing a total field imaging of the sub-soil surrounding a hole, whichimproves the performances of the present tools, on the one hand in theeffectively total coverage of the periphery of the wall up to diametersof at least 216 mm and even beyond, with a definition of vertical andhorizontal measurement of the order of a centimeter, over the wholesurface of the wall, on the other hand, in the maximum elimination ofthe parasitic effects of the fluid medium existing in the hole and,finally, allowing its rise during measurement, limiting the possiblevibrations due to the frictions and despite the presence of the ambientfluid medium, which must therefore circulate on either side of theprobe.

SUMMARY OF THE INVENTION

One solution to the problem raised is a total field imaging probe foreffecting imaging loggings over the whole periphery of the wall of ahole bored in the sub-soil and filled with a fluid, because of sensorsfor measuring the characteristics of this soil, mounted on or in thebody of said probe, certain parts of this body being able to come intocontact with said wall and in rest position, its outer shape being thatof a cylinder with a diameter smaller than the diameter of said hole:according to the invention, said body of the probe comprises at leastone casing moulded from a material of elastomer type, comprising helicalelements, separated by bellows, with the result that, thanks to anyinternal expansion means, said casing unfolds so that said elementsfollow the shape of the wall of the well and that said bellows ensurethe connections between these elements and maintain the assembly of thecasing in semi-rigid manner following a continuity of its peripheralface.

In different embodiments, said casing may be completely closed andcontains a fluid which may be placed under pressure in order toconstitute said internal expansion means of the casing against the wallof the well, said helical elements being provided with appropriateelastic return systems to return into rest position in the absence ofrelative pressure of the fluid greater than that of the ambient fluidand said bellows leave a sufficient passage for said fluid during therise of the probe during measurement; or said internal expansion meansis constituted by any hydromechanical system connecting each helicalelement to a static structure element of the probe body.

In an application of acoustic measurement, at least one of the sensorsis an ultrasonic transducer, located inside said probe, said casingbeing completely closed and filled with a fluid allowing a good acousticcoupling.

In another application of electrical measurement, possibly combined withthe preceding one, at least certain of said sensors are knownmicroresistive measurement electrodes integrated and moulded in saidhelical elements able to come into contact with said wall of the well,and disposed along these elements in order that, whatever the diameterof the position of expansion and of measurement compatible with saidprobe, the whole of the periphery of the wall of said well is entirelycovered and investigated by said electrodes, when said probe is raisedwithout rotation.

Other different applications of measurements are also possible.

The result is a novel total field imaging probe which considerablyimproves the performances of the present tools and opens up thepossibility of numerous uses. In fact, because of the combination ofhelical elements which may come into contact with the wall of the holeor of the well, and of bellows inserted between these elements, theassembly constituting a semi-rigid body which may therefore well supportthe effects of friction against the wall, it is possible to adapt alltypes of known sensors usable for loggings; these technical solutionsproposed are even more interesting when these sensors necessitate a goodcontact with the formation of the sub-soil to be investigated.

This probe according to the invention, in fact, solves in priority theproblem of coverage of the wall of the well or of measurements ofmicroresistivity or any measurement using sensors in contact with thiswall. However, the principle of this probe also allows applicationsand/or improvements for other methods of imaging loggings, such as thosedescribed by way of example and in greater detail in the followingdescription and concerning:

acoustic imaging, as the probe makes it possible to overcome theservitude due to the quality of the mud and considerably to reduce theincidence of the others (contrasts of the acoustic impedances, etc. . .);

the combination with the electrical imaging of an acoustic system alsoallowing the exact measurement of the geometry of the hole;

the problem of the well camera in an opaque medium, such as the drillingmud, which generally prevents use thereof apart from water andlow-energy geothermic boreholes.

Another advantage, and not the least, whatever the application of thisprobe, is that the latter, being given its principle and the materialsused for making it, is extremely light compared with conventionalprobes.

In terms of measurement, this means:

better application of the elements and therefore of the measuringsegments when these elements bear sensors;

better centering of the probe in deviated wells;

reduction of the "YO-YO" effect of the probe, which is a non-lineardisplacement due to a variable effort of friction and which isencountered in the systems comprising shoes.

Other advantages of the present invention may be mentioned, but thosecited above already show sufficient to demonstrate the novelty andinterest.

BRIEF DESCRIPTION OF THE DRAWINGS

The description, drawings and Figures hereinafter represent anembodiment of the invention, but have no limiting character; otherembodiments are possible from the Claims which specify the scope andextent of this invention.

FIG. 1 is a side view of an embodiment of probe according to theinvention in rest position penetrating in a hole.

FIG. 2 is a side view of the probe of FIG. 1 in position formeasurement.

FIG. 3 is a view in section along AA in FIG. 2 of a probe in positionfor measurement.

FIG. 4 is a perspective view of another embodiment of probe.

DETAILED DESCRIPTION

The present invention concerns the probe itself, its principle ofunfolding and of production and in no way the electronics associatedwith the different measurements which have already been developed forthis type of applications and which are therefore not described.

In its rest, or folded, position, as shown in FIG. 1, the probe 5 may bedisplaced in the bored hole 1, its outer shape being that of a diameterless than that of said hole 1. The latter is generally filled with fluid3 which, in the case of a borehole underway, is opaque mud, and whichtherefore separates the wall 2 of the hole from the probe body 5; thisfluid must be able to pass from one side of the probe to the other whenthe latter is unfolded as in FIG. 2 and displaced linearly duringmeasurement.

Said probe body 5 comprises according to the invention a casing 6,moulded from a material of elastomer type, resistant to abrasion, oils,heat, etc. . . This casing comprises helical elements 7 which, when theyare in rest position, are virtually adjacent one another: these elements7 may comprise, for the applications of measurement necessitatingcontact with wall 2, sensors 4 such as electrodes or transducers, whichare then distributed helically over said casing 6.

In that case, it may be envisaged to cover simply zones 7 in directcontact with the wall of the well during the unfolding, apart from thesensors, with a small thickness of material which particularly resistsabrasion, without being detrimental to the necessary suppleness. Theassembly of the electrodes or transducers may be moulded in the materialof the probe body, in accordance with well known techniques which areused elsewhere.

In a variant, the material of the electrodes may be elastomer or likeconductive material.

FIG. 2 shows the same probe 5 in position for measurement, as a resultof expansion and unfolding of its casing 6 against said wall 2 of thewell or hole 1.

The radial elasticity, necessary for unfolding, is obtained via bellows8 which may be disposed in accordance with FIG. 3. Furthermore, when thecasing 6 is tight and closed, the section available between thesebellows 8 and the wall of the well 2 allows passage of the fluid such asthe drilling mud 10, during rise of the probe 5 in the course ofmeasurement.

Whatever the expansion adapted to the diameter of the hole 2, saidexpansion makes it possible, up to a maximum diameter of the wall whichis set and particular to each type of probe, to ensure a coverage "d" ofmeasurements between the sensor 4 located at the top of an element 7 andthat located at the bottom of the adjacent element, during the rise ofthe probe. Depending on the desired precision and the definition of themeasurement, there will be set a negative minimum value of "d" in orderto have a guaranteed surface coverage for the two sensors and a positiveminimum value when a tolerance of non-coverage is accepted.

The upper and lower parts of the zones of measurement of the casing 6may be protected in order to avoid the probe hooking on descending or,in the course of measurement, on rising. This protection may beobtained, for example by a system of "petals" which overlap more or lessas a function of the diameter.

In a preferred embodiment of the invention, unfolding takes place byplacing under pressure a fluid 10 inside the tight casing 6, which mustthen be closed, completely closed and tight, controlled by a mechanicalreturn system 9 to return into rest position in the absence of relativepressure of the fluid 10, ensuring for the helical elements 7 a knowngeometry as a function of the outer diameter of the cylinder on whichthese elements are positioned and which corresponds, in position ofmeasurement, to that of said hole 1, as shown in FIG. 3.

In another embodiment, said internal expansion means 9 is constituted byany hydromechanical system connecting each helical element 7 to a staticstructure element of the probe body 5; this hydromechanical system maybe constituted by jacks and springs.

According to another embodiment, the casing 6 comprises a double wall,of which the space thus defined may be placed under pressure in order toensure greater rigidity.

Thus, said casing 6 and its internal expansion means 9 may be made inaccordance with one of the forms described hereinabove or may combineseveral of these forms, such that:

either the casing 6 is closed and tight, and may be unfolded by a fluidunder pressure;

the casing 6 is closed and tight and may be unfolded by an internalmechanical or hydromechanical means 9 enclosed in said casing, which maythen be filled with a liquid 10 then remaining under equal pressure withthe outside atmosphere 3;

the casing 6 is open upwardly and downwardly and articulated arms 13ensure unfolding thereof, or any other mechanical or hydromechanicalmeans;

or the envelope is as in the preceding case and comprises, in addition,a double wall to render it more rigid.

FIGS. 1 and 2 illustrate forms in accordance with the first two examplesmentioned hereinabove. The casing 6 protects all of the internal volumethus defined from the drilling mud. This implies that the upper andlower openings of this casing are tight on the corresponding parts ofthe body of the probe 5.

However, the two basic characteristics which are that of having asupport on the one hand, ensuring a helical distribution of segment ableto bear sensors and, on the other hand, presenting a capacity of radialunfolding, because of the presence of bellows between the zones ofmeasurement, may also be ensured without necessitating a closed tightcasing on the probe body; this latter possibility is shown in FIG. 4, inillustration of the forms in accordance with the latter two examplescited above.

In this Figure are found again: the segments 7 with helically implantedzones of measurement and the bellows 8.

In the case shown, articulated arms 13 ensure unfolding from the supportor probe body 5; the electrodes and the assembly of the necessaryelectrical connections may be moulded in the casing 6 at the level ofthe zones of measurements.

In a variant, therefore, the casing 6 may be of double thickness andhollow, the interior being filled with a hydraulic fluid under equalpressure in rest position, and in slight overpressure during themeasurements with the ambient fluid, the overall geometry of the systemremaining identical.

One of the objectives envisaged by this latter variant is that ofintroducing a variable rigidity at the level of the zones of measurementand of being able to modify the radius of curvature thereof byincreasing the pressure inside, so as to be adapted to the variations ofthe diameter of the borehole.

In this variant, the bellows 8 are not hollow but some tubular passagesof small diameter made in the thickness of the bellows ensure hydrauliccontinuity between the different zones of measurements.

The electrical connections between the electronics inside the probe 5and the electrodes pass behind the unfolding arms 13 and penetrate inthe probe via tight connectors.

The same applies to the hydraulic connections in the case of hollowzones of measurements 7.

The probe comprises mechanical means 9 necessary for regular folding,such as described hereinabove; any other solution is possible, such as,for example, a spring system mounted on the inner edge of the bellows 8.This return assembly between the bellows and the static inner part ofthe probe 5 allows a position of equilibrium of the bellows between thisreturn tension and the pressure differential existing between theinterior of the probe and the well, in the closed casing and inner fluidunder pressure version.

The helical elements 7, which may therefore be for measurement, may bemade of one or more segments as a function of the unfolding to beobtained, the only servitude being that of knowing at any moment thegeometry of these measuring elements as a function of the unfoldeddiameter.

The theoretical calculation of coverage takes into account an ascendinglinear displacement of the probe 5, without rotation of the latter inthe borehole, which is rarely the case.

The strains governing this problem of rotation have two origins:

strain existing between the two layers of strands in the cable ensuringmechanical connection for handling and transmission of the measurementstowards the surface;

strain due to the helical elements themselves and tending, in theabsence of opposing force, to rotate the probe following their ownpitch.

If the latter effect is preponderant, likewise helical recesses having apitch in a direction opposite that of the helix may be moulded betweenthe zones of measurement in order to oppose this action.

Determination of the diameter may be made in several ways, depending onthe degree of precision desired:

with the well known technique using an internal potentiometer along adiameter, of which the slide moves as a function of the unfolding of thediameter;

by measuring the time of passage of an acoustic wave emitted by a sensor4₁, which in that case is an ultrasonic transducer 4₁ located insidesaid probe 5, said casing 6 being completely closed and filled with afluid 10 allowing a good acoustic coupling.

At least one or some acoustic reflectors are placed regularly, whenthere are several, inside the elements 7 of the probe and, because ofsaid acoustic sensor 4₁, in that case turning and rotative, when thereare several, and of which the wave 12 is reflected by each of thesereflectors 11, enable the geometry of the hole 1 to be perfectly known.

In fact, the speed of sound in the fluid within the probe beingperfectly known, as well as the thickness of material between the wallof the well and the inner surface of the reflector, the radius may bedetermined permanently.

This technique with respect to the use of the wave reflected by the wallmakes it possible to obtain a much greater amplitude which, furthermore,is independent of the lithology.

In another embodiment, said acoustic reflectors 11 are located at theintersection of each half-bellows 8 and oriented so that, whatever theunfolding, the perpendiculars to the planes of these reflectors passthrough the acoustic sensor 4₁, of which the wave 12 reflected by eachof the reflectors always enables the geometry of the hole 1 to be known.

As stated previously, the geometry of the probe being known as afunction of the diameter, the position of the sensors 4 such aselectrodes or transducers with respect to fixed references of the probe,is permanently known.

The position of the probe in space is known because of a navigationmodule constituted for example by a three-axis accelerometer and athree-axis magnetometer or a three-axis gyrometer, of which theprinciples and use are perfectly well known.

In the acoustic imaging effected in the configuration describedpreviously, but for example beneath or outside of the acousticreflectors in order to know and investigate directly the wall 2 of thewell, the path of the acoustic wave is as follows:

passage in the fluid 10 of the probe body 5;

passage through the sheath of the casing 6;

passage in the drilling fluid 3;

reflection on wall 2;

passage in the drilling fluid 3;

passage through the sheath of the casing 6;

passage in the fluid of the probe body 10;

detection by the sensor 4₁.

The amplitude of the wave reflected and detected by the sensor may bealtered by different parameters:

contrasts between the various acoustic impedances of the materials orfluid traversed: fluid of the probe/casing, casing/drilling fluid, etc.. . ;

density of the mud and particularly presence of solid microparticleswhich contribute in preponderant manner to the weakening of the acousticsignal;

excentering of the tool in the hole.

The use of a probe similar in the principle of unfolding to thatdescribed previously but using, as internal fluid 10 and as material ofthe helical sectors 7, constituents chosen in particular by reason oftheir appropriate acoustic impedance, makes it possible to overcome theconstraint due to the quality of the mud and considerably to reduce theincidence of the others.

The incidence of the excentering of the probe on the measurement ofamplitude being able to be calculated, the measurement of the positionof the probe according to the principle explained previously makes itpossible, after processing on the surface of the measurement by thecorresponding algorithm, to restore the corrected amplitude.

In that case, nothing, apart from the density of information to beraised to the surface, prevents different imaging measurements, forexample acoustic and microresistive, to be coupled on the same probe;said probe is in that case constituted by at least two casings 6 placedone above the other and able to comprise elements 7 and sensors 4 ofcharacteristics different from one another in order to be able to makedifferent types of measurement. The general geometry of the proberemains unchanged, the upper part being reserved for one type ofmeasurement, the lower part for another, and so on, if there areseveral.

Finally, this probe may include a camera: in that case, the material ofthe elements 7 is transparent, said casing 6 being completely closed andfilled with a fluid allowing a good optical coupling, one of saidsensors 4 placed in the casing such as for example acoustic sensor 4,shown in FIG. 3, being a well camera which may allow observation even inthe presence of opaque mud.

The "CCD" well camera, of which the technique is well known, may beoriented radially about 360° of freedom, its axis of rotation beingaxially displaceable in order to be able to investigate over the wholeheight of the window defined by that of the transparent elements.

This type of measurement is generally effected in stationary manner andthe probe is folded between each station. Cleaning of the walls with theaid of appropriate scrapers with the boring rods, may be effected beforethe measurements.

I claim:
 1. Total field imaging probe for effecting imaging logs overthe whole periphery of the wall of a hole bored in the sub-soil andfilled with a fluid said probe including sensors for measuring thecharacteristics of this soil mounted on or in the body of said probe,certain parts of said body being able to come into contact with saidwall and, in the rest position, having an outer shape of a cylinder anda diameter smaller than the diameter of said hole, said body of theprobe comprising: at least one casing molded from a material ofelastomer type, said casing comprising helical elements separated bybellows responsive to internal expansion means for unfolding said casingwhereby said elements follow the shape of the wall of the well, saidbellows ensuring the connections between the elements and maintainingthe assembly of the casing in semi-rigid manner for following aperipheral surface continuity.
 2. Total field imaging probe according toclaim 1, wherein said casing is completely closed and contains a fluidwhich may be placed under pressure, in order to constitute said internalmeans for expanding the casing against the wall of the well, saidhelical elements being provided with appropriate elastic return systemsfor returning to a rest position in the absence of relative pressure ofthe fluid greater than that of ambient fluid and said bellows leave asufficient passage for said fluid during the raising of the probe in thecourse of measurement.
 3. Total field imaging probe according to claim1, wherein said internal expansion means is constituted by anyhydromechanical system connecting each helical element to a staticstructure element of the probe body.
 4. Total field imaging probeaccording to claim 3, wherein the casing comprises a double wall, ofwhich the space thus defined may be placed under pressure to ensure agreater rigidity.
 5. Total field imaging probe according to claim 1,wherein at least certain f said sensors are known microresistivemeasurement electrodes integrated and moulded in said helical elementswhich may come into contact with said wall of the well, and disposedalong these elements so that, whatever the diameter of the position ofexpansion and of measurement compatible with said probe, the whole ofthe periphery of the wall of said well is entirely covered andinvestigated by said electrodes, when said probe is raised withoutrotation.
 6. Total field imaging probe according to claim 1, wherein atleast one of said sensors is an ultrasonic transducer, located insidesaid probe, said casing being completely closed and filled with a fluidallowing a good acoustic coupling.
 7. Total field imaging probeaccording to claim 6, wherein acoustic reflectors are placed in regularmanner inside the elements of the probe for determining the geometry ofthe hole when said casing is rotating by measuring acoustic wavesreflected from said reflectors to said acoustic sensor.
 8. Probeaccording to claim 6, wherein acoustic reflectors are located at theintersection of half-bellows and oriented for directing linesperpendicular to the planes of these reflectors to pass through theacoustic sensor, the wave reflected by each of the reflectors enablingthe geometry of the hole to be known.
 9. Probe according to claim 1,wherein the material of the elements is transparent, said casing beingcompletely closed and filled with a fluid allowing a good opticalcoupling, one of said sensors being a well camera which may allowobservation even in the presence of opaque mud.
 10. Total field imagingprobe according to claim 1, wherein it is constituted by at least twocasings placed one above the other and able to comprise elements andsensors of characteristics different from one another in order to beable to effect different types of measurement.